Several devices are known for the fusion of adjacent vertebrae, among which:                instrument-assisted fusion devices, comprising rods and screws for fixing the relative position of the vertebrae;        “cage”-type fusion devices, which can be implanted by an anterior or is posterior or lateral approach;        interspinous fusion devices, suitable for positioning between the spinous processes of two adjacent vertebrae.        
US2005/0055094 discloses a cage-type fusion device known as “Optimesh”. This device is suitable for insertion between two vertebral bodies to replace a damaged intervertebral disc, and comprises a closed cage element that is manufactured starting from a fibrous material. The cage element is filled with a granular bone material. A drawback of this device is that the fusion between the granular bone material and the vertebral bodies must take place through the fibrous material of the cage element, and is therefore difficult to obtain. Moreover, in case of determined stress conditions, the cage element can explode. Furthermore, the device requires screws and rods, in order to fix the two adjacent vertebrae to each other, and to provide a primary stabilization; therefore it belongs to the instrument-assisted fusion devices, which are more difficult to be manufactured and implanted.
A cage-type device is also known as “Biocage”, which is conceived for replacing a cervical vertebral disk and for providing a fusion of the vertebral bodies adjacent to the cervical vertebral disk. The device comprises a substantially tubular net cage element and a granular bone material arranged in the cage element. The cage element is open below and above so as to allow a contact of the granular material with the surface of the two vertebrae to be fused together. The cage element may be made of PET coated by a stiff reabsorbible polymer, e.g. PLLA.
Also this type of device very often fails to form a spinal fusion. As it is known, in fact, osteosynthesis requires that suitable forces are exerted on the bone material. Since the cage element is stiff, the loads of the vertebrae are to applied to the cage element instead of the granular bone material. Therefore, the bone material of the device is substantially free from mechanical stress and so it is not stimulated for fusing with the bone of the vertebrae.
Moreover, this device has the drawback of triggering foreign body reactions, because it comprises an excessive amount of polymer, although reabsorbible, in order to obtain a stiff structure.
The interspinous fusion devices limit the sole flexion-extension movement of the spine and only partially. On the contrary, the interspinous fusion devices do not substantially limit the relative lateral bending and torsion movements. Therefore, the interspinous fusion devices leave to the adjacent vertebrae a residual relative mobility, which disturbs or even hinders the fusion. In other words, the exclusive use of interspinous fusion devices cannot provide a primary stabilization that develops into a fusion.
Accordingly, the need is felt of a device for fusion of two adjacent vertebrae comprising a bone fusion material that, once implanted, enables the two vertebrae to mechanically stimulate the fusion of the bone material and that hinders the relative mobility of the two vertebrae immediately after implanting the device, providing a primary stabilization required for the fusion to develop, overcoming the drawbacks of the prior art.
US 2008/167686 A1 describes an intervertebral spacer device comprising a container consisting of two compression resilient elements, in particular leaf springs. In an exemplary embodiment, the container has two open end portions. A procedure is also described in which the spacer device is filled with a material adapted to promote bone growth, substantially for fixing it in its implant site.
WO2009/005819 describes an intervertebral device for providing a “mobile fusion” of two adjacent vertebrae, i.e. for forming a connection of the vertebrae in which an elastic relative mobility is enabled therebetween. In fact, exemplary embodiments are described that are expressly conceived to allow at least two vertebral movement modes selected among the flexion-extension movement, the lateral bending movement and the axial rotation movement.
Both devices include contact portions, designed to come into contact with the vertebral bodies, which resiliently react to the forces exerted by the vertebral bodies. The bone growth promoting material cannot do more than fixing the contact portions to the respective vertebral bodies, but it cannot stiffen the device and, therefore, it cannot immobilize the two vertebrae to each other. In fact, in the former case, the compression movement of the container does not enable a primary stabilization and, accordingly, the fusion of the two vertebral bodies. In the latter case, the relative mobility of the vertebral bodies is expressly wanted.
Therefore, both such devices embody substantially an intervertebral disc prosthesis and are not used to form a vertebral fusion in the true sense.
It is also observed that the container of US 2008/167686 A1 cannot be filled with an osteosynthesis material, in particular with bone material, in the granular form.